This invention relates to an air/fuel ratio feedback control system for internal combustion engines, and more particularly to fail safe device for detecting a failure in such system and performing necessary fail safe actions.
An air/fuel ratio feedback control system for performing feedback control of the air/fuel ratio of an air/fuel mixture being supplied to an internal combustion engine has already been proposed by the applicants of the present application, which comprises a sensor for detecting the concentration of an exhaust gas ingredient emitted from the engine, an air/fuel ratio control valve of which the valve body position determines the air/fuel ratio of the mixture being supplied to the engine, an actuator arranged to displace the actuator in a continuous manner in response to an output signal generated by the sensor, and reference position detecting means for generating a reference position signal when the actuator passes a reference position which is provided between two opposite operating positions mechanically limiting the movable range of the actuator.
In principle, the above air/fuel ratio feedback control system proposed by the applicants is adapted to perform a feedback control operation in such a manner that the concentration of the exhaust gas ingredient is detected by means of the above sensor which may be comprised of an O.sub.2 sensor provided in the exhaust pipe of the engine, and an electronic control circuit (e.g., ECU) operates on the output signal of the sensor to drive the actuator which may be comprised of a pulse motor for control of the air/fuel ratio control valve so as to achieve a proper air/fuel ratio of the mixture being supplied to the engine.
In addition to the above O.sub.2 sensor, the air/fuel ratio feedback control system is provided with sensors for detecting the operating condition of the engine, which include an engine rpm sensor, an atmospheric pressure sensor, an intake pipe-absolute pressure sensor and an engine coolant temperature sensor. Responsive to output signals of these sensors, the electronic control circuit proceeds with a predetermined program to determine the fulfillment of a closed loop control condition and open loop control conditions and produce a control signal corresponding to a fulfilled control condition for driving the actuator. As for the O.sub.2 sensor, the electronic control circuit has a circuit for detecting the activation of the O.sub.2 sensor at the start of the engine, whereby determination is made as to the fulfillment of a condition of initiation of the air/fuel ratio control on the basis of an activation signal generated by the above detecting circuit and an output signal indicative of a value exceeding a predetermined value generated by the engine coolant temperature sensor.
The actuator or pulse motor is provided with a reference position detecting device which is comprised e.g. of a reed switch which is adapted to supply a reference position signal to the electronic control circuit when the actuator passes a predetermined reference position. The electronic control circuit is in turn responsive to this reference position signal to replace an actual actuator position value stored therein by a reference position value, this always accurately memorizing the actual position of the actuator for achievement of accurate air/fuel ratio control.
However, in the event of occurrence of abnormality in the output signals of these sensors due to a failure in the sensors or the reference position detecting device or a failure in the related wiring system, the electronic control circuit is unable to properly determine the fulfillment of the open loop control conditions and the closed loop control condition in response to the actual operating condition of the engine so that the resulting air/fuel ratio has an abnormal value, which can spoil the driveability and exhaust gas emission characteristics of the engine or cause a misfire in the engine.
The air/fuel ratio feedback control system can malfunction not only due to failure in the sensors or the reference position detecting device but also due to many other factors. If no emergency measure is taken in the event of occurrence of a failure in the air/fuel ratio feedback control system, sometimes one would not imagine what position the actuator will be controlled to. Let it now be assumed that in the event of occurrence of a failure in the system when the associated automotive vehicle is running in a location at high elevation, the actuator is stopped at a position RICH MAX corresponding to a minimum of air/fuel ratio, the resulting mixture is extremely rich, which greatly deteriorates the driveability of the engine and sometimes makes it impossible to drive the engine.
To avoid the above disadvantages, the air/fuel ratio feedback control system is provided with a fail safe function such that upon occurrence of a failure in the aforementioned sensors, the actuator is automatically moved to a particular position, namely, a predetermined idle position which is compensated for atmospheric pressure, and held there.
However, there can be a disagreement between the actual position of the pulse motor and a count in an actual pulse motor position counter provided in the electronic control circuit due to skipping or racing of the pulse motor or other factors. If a failure occurs in the system when the above disagreement exists, the pulse motor is not moved to and held at the aforementioned predetermined idle position with accuracy.